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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Per Share Operating Performance: |  |  |
Net asset value, beginning of year | $ 9.20 | $ 16.91 | $ 24.35
Total income (loss) | 3.33 | (7.55) | (7.16)
Total expenses | (0.14) | (0.16) | (0.28)
Net increase (decrease) in net asset value | 3.19 | (7.71) | (7.44)
Net asset value, end of period | $ 12.39 | $ 9.20 | $ 16.91
Total Return | 34.67% | (45.59%) | (30.55%)
Ratios to Average Net Assets |  |  |
Total income (loss) | 6.56% | (49.07%) | (24.25%)
Management fees | 0.60% | 0.60% | 0.60%
Expenses excluding management fees | 0.51% | 0.75% | 0.73%
Net income (loss) | 5.45% | (50.42%) | (25.58%)

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